The present invention relates to a circuit arrangement for production of a rest signal after a supply voltage has fallen and risen again, in particular for use in a circuit which has a static memory.
Static memories (SRAMs) permanently require a supply voltage for reliable maintenance of a predetermined memory state, and this supply voltage must not fall below a predetermined value which is dependent on the technology of the respective memory. If the supply voltage falls below this minimum value despite this, then this can lead to undesirable changes in the memory states of individual memory cells, or to undefined memory states.
It is therefore necessary to identify such a fall in the supply voltage (which, for example, may be also be caused by EMC interference radiation) and to make a reset signal available once the supply voltage has risen again to a value which is required for reliable operation. A reset signal such as this may be used, for example, to carry out initialization of the memory via further circuit components. Various concepts are known for production of such reset signals.
Fundamentally, it is possible to compare the supply voltage with a reference voltage continuously and to produce a reset signal when the supply voltage falls below the reference voltage. A circuit arrangement such as this, which uses a reference voltage source, for producing a reset signal is described, for example, in U.S. Pat. No. 5,552,725.
This concept has the disadvantage that the reference voltage source which, for example, may also be in the form of a band gap reference voltage source, permanently draws an operating current in the μA range. This concept is not suitable for battery-powered circuit applications in which a minimum power consumption must be ensured in order to achieve a long battery life.
One example of applications such as these is so-called ultra low power sensors, whose voltage supply is intended to be ensured, by means of a small battery, for a time period of three to ten years. Sensors such as these operate intermittently with short operating phases during which the mean current that is drawn is in the μA range, and with rest phases which are long in comparison to the operating phases and in which the current that is drawn is intended to be less than 20 nA, in order to ensure a long battery life. Reset circuits which draw small currents such as these can be produced, for example, using very high-value resistors, but these occupy a very large amount of space. In the case of sensors such as these, comparison values relating to the measured values determined by the sensor are stored, for example, in an SRAM. Sensors such as these are used, for example, as tire pressure sensors in motor vehicles, which cannot be connected to the vehicle power supply system voltage and which must therefore be supplied via their own battery, which is isolated from the vehicle power supply system battery.
U.S. Pat. No. 6,556,058 B2 describes a POR circuit (POR=Power On Reset) for production of a reset signal after a supply voltage has been switched on, in which a threshold voltage of an MOS transistor is used as a reference voltage for the production of a reset signal when the supply voltage rises.
U.S. Pat. No. 6,600,350 B2 describes a POR circuit having a capacitor which is connected to the supply voltage via a resistor, and whose state of charge is evaluated in order to produce a reset signal. This circuit has the disadvantage that it does not operate reliably when the supply voltage rises slowly again.
The aim of the present invention is to provide a circuit arrangement for production of a reset signal after a supply voltage has fallen and risen again, which circuit arrangement draws a very low current, can be produced easily and operates reliably.